A number of serious medical conditions may be treated in a minimally invasive manner with various kinds of catheters designed to reach treatment sites internal to a patient's body. One such medical condition is atrial fibrillation—a serious medical condition that results from abnormal electrical activity within the heart. This abnormal electrical activity may originate from various focal centers of the heart and generally decreases the efficiency with which the heart pumps blood. It is believed that some of these focal centers reside in the pulmonary veins of the left atrium. It is further believed that atrial fibrillation can be reduced or controlled by structurally altering or ablating the tissue at or near the focal centers of the abnormal electrical activity.
One method of ablating tissue of the heart and pulmonary veins to control atrial fibrillation includes delivering radiofrequency (RF) energy to the tissue to be ablated. In particular, high frequency energy can be employed, for example, to cause ionic agitation and frictional heat in targeted tissue, causing permanent damage to the tissue. Once damaged, the tissue may no longer propagate or source electrical signals, and the fibrillation may be treated or reduced. The RF energy can be delivered by an RF catheter having an RF source at a distal treatment end that is positioned at a treatment site inside a patient during a treatment procedure.
Another method of ablating tissue of the heart and pulmonary veins to control atrial fibrillation is through cryotherapy, or the extreme cooling of body tissue. Cryotherapy may also cause permanent alteration to treated tissue, preventing the treated tissue from propagating or sourcing electrical signals, thereby reducing or eliminating atrial fibrillation. Cryotherapy may be delivered to appropriate treatment sites inside a patient's heart and circulatory system by a cryotherapy catheter. A cryotherapy catheter generally includes a treatment member at its distal end, such as an expandable balloon having a cooling chamber inside. A cryotherapy agent may be provided by a source external to the patient at the proximal end of the cryotherapy catheter and delivered distally through a lumen in an elongate member to the cooling chamber where it is released. Release of the cryotherapy agent into the chamber cools the chamber, the balloon's outer surface, and tissue that is in contact with the outer surface, to perform ablation. The cryotherapy agent may be exhausted proximally through an exhaust lumen in the elongate member to a reservoir external to the patient.
In addition to facilitating permanent tissue alteration, cryotherapy facilitates temporary electrical inactivation of tissue in a manner that enables a physician to test the likely results of ablation through a reversible process. Such a process is commonly referred to as cryomapping, and generally involves cooling tissue to near freezing (e.g., to 0° C.) but well above a temperature at which the tissue would be ablated (e.g., −20° C.).
It may be advantageous to map the electrical activity of a pulmonary vein (or other treatment site) prior to permanent ablation by either RF or cryotherapy, in order to pinpoint appropriate ablation target sites. Some apparent target sites may not actually contribute to abnormal electrical activity, and treating such sites may not be desirable. Treating other target sites may affect healthy tissue in undesirable ways (e.g., creating conduction blocks). Precisely mapping the electrical activity in a target treatment region can help focus the treatment and confirm its efficacy and safety.
Various specialized mapping catheters may be employed to electrically map tissue, such as a circular catheter or a multi-electrode basket catheter. Such mapping catheters can be positioned at possible treatment sites inside a patient, and electrodes at those sites can provide signals to a processing system external to the patient that can process the signals and provide physicians with information to subsequently position a separate RF or cryotherapy catheter and deliver with that separate catheter appropriately targeted ablation energy.